The R-T-B based sintered magnet (R is rare earth element(s), T is one or more transition metal elements with Fe as necessity, and B is boron) with the tetragonal compound R2T14B as the main phase is known to have excellent magnetic properties and has been a representative permanent magnet with high performance since the invention in 1982 (Patent document 1).
Especially, the R-T-B based sintered magnets with the rare earth element(s) R being consisted of Nd, Pr, Dy, Ho and Tb have a large anisotropic magnetic field Ha and are widely used as permanent magnet materials. Among them, the Nd2Fe14B based sintered magnet having Nd as the rare earth element R is widely used in livelihood, industries, and transportation equipments because it has a good balance among saturation magnetization Is, Curie temperature Tc and anisotropic magnetic field Ha. However, it is well known that the corrosion resistance of the R-T-B based sintered magnet is relatively low because the magnet contains rare earth elements as the main component.
Herein, the mechanism of the corrosion was considered as follows. First, if the water from the water vapor or the like in a use environment adheres to the surface of the sintered magnet, a cell reaction occurs due to a potential difference generated between the main phase and the grain boundary phase. In this process, hydrogen is produced. The produced hydrogen is stored in an R-rich phase and thus the R-rich phase is changed to hydroxide. In addition, in the cell reaction of the water and the R-rich phase stored with hydrogen, hydrogen in an amount more than that stored in the R-rich phase is produced. Accompanying with the reaction, the volume of the grain boundary part expands so that main phase grains are caused to fall off. As the result, a newly formed surface of the R-T-B based sintered magnet emerges and the reaction progresses inside the magnet.
With respect to the problem, in Patent Document 2, a method is proposed to improve the corrosion resistance by homogeneously dissolving a specified amount of Co as a solid-solution in the R-rich phase. This is considered that the potential difference between the main phase and the grain boundary phase becomes small and the cell reaction is suppressed effectively by dissolving Co as the solid-solution in the R-rich phase. Thus, the corrosion resistance is improved.